4,984 research outputs found
Tentative Detection of the Rotation of Eris
We report a multi-week sequence of B-band photometric measurements of the
dwarf planet Eris using the {\it Swift} satellite. The use of an observatory in
low-Earth orbit provides better temporal sampling than is available with a
ground-based telescope. We find no compelling evidence for an unusually slow
rotation period of multiple days, as has been suggested previously. A
1.08 day rotation period is marginally detected at a modest level of
statistical confidence (97%). Analysis of the combination of the
data with the ground-based B-band measurements of \citet{2007AJ....133...26R}
returns the same period (1.08 day) at a slightly higher statistical
confidence (99%).Comment: Accepted to Icarus 2008-Aug-19. 19 pages total, including 4 figures
and 1 tabl
Tunneling and Non-Universality in Continuum Percolation Systems
The values obtained experimentally for the conductivity critical exponent in
numerous percolation systems, in which the interparticle conduction is by
tunnelling, were found to be in the range of and about , where
is the universal conductivity exponent. These latter values are however
considerably smaller than those predicted by the available ``one
dimensional"-like theory of tunneling-percolation. In this letter we show that
this long-standing discrepancy can be resolved by considering the more
realistic "three dimensional" model and the limited proximity to the
percolation threshold in all the many available experimental studiesComment: 4 pages, 2 figure
Hysteresis loops of Co-Pt perpendicular magnetic multilayers
We develop a phenomenological model to study magnetic hysteresis in two
samples designed as possible perpendicular recording media. A stochastic
cellular automata model captures cooperative behavior in the nucleation of
magnetic domains. We show how this simple model turns broad hysteresis loops
into loops with sharp drops like those observed in these samples, and explains
their unusual features. We also present, and experimentally verify, predictions
of this model, and suggest how insights from this model may apply more
generally.Comment: 4.5 pages, 5 figure
Low-Temperature Permittivity of Insulating Perovskite Manganites
Measurements of the low-frequency (f<=100 kHz) permittivity and conductivity
at T<= 150 K are reported for La(1-x)Ca(x)MnO(3) (0<=x<=1) and
Ca(1-y)Sr(y)MnO(3) (0<=y<=0.75) having antiferromagnetic, insulating ground
states covering a broad range of Mn valencies from Mn(3+) to Mn(4+). Static
dielectric constants are determined from the low-T limiting behavior. With
increasing T, relaxation peaks associated with charge-carrier hopping are
observed in the real part of the permittivities and analyzed to determine
dopant binding energies. The data are consistent with a simple model of
hydrogenic impurity levels and imply effective masses m*/m_e~3 for the Mn(4+)
compounds. Particularly interesting is a large dielectric constant (~100)
associated with the C-type antiferromagnetic state near the composition
La(0.2)Ca(0.8)MnO(3).Comment: 6 pages, 8 figures, PRB in pres
Further studies of space-time clustering of Burkitt's lymphoma in Uganda.
All hospital-treated cases of Burkitt's lymphoma (BL), with onset of symptoms in the period 1963-68 and resident in the Lango and Acholi districts of Uganda, were identified. The average annual incidence of BL in the 6-year period was 1-87 X 10(-5), similar to that in the adjacent West Nile district. Contrary to findings in other areas of Uganda, there was no evidence of seasonal variation in the onset of cases, nor of space-time clustering, nor of a decline in the incidence of BL in the study period. An inverse relationship was noted between the median age at onset of BL and the incidence of the disease in different areas of Uganda, a finding consistent with intense malarial infection being a precipitating factor for BL. The variable observations with respect to space-time clustering of BL and seasonal variation in incidence in different areas remains unexplained, but it is suggested that a closer study of the patterns of malarial infection in these areas may help to account for the findings
Elasticity of Stiff Polymer Networks
We study the elasticity of a two-dimensional random network of rigid rods
(``Mikado model''). The essential features incorporated into the model are the
anisotropic elasticity of the rods and the random geometry of the network. We
show that there are three distinct scaling regimes, characterized by two
distinct length scales on the elastic backbone. In addition to a critical
rigidiy percolation region and a homogeneously elastic regime we find a novel
intermediate scaling regime, where elasticity is dominated by bending
deformations.Comment: 4 pages, 4 figure
Kaposi's Sarcoma in Uganda: Geographic and Ethnic Distribution
Over the quinquennium 1964-68 the crude annual incidence of Kaposi's sarcoma in Uganda per million of the population was 7·9 overall, 14·6 for males and 1·1 for females. Statistical analysis indicates that the disease is most prevalent in highland areas to the west and among the indigenous Bantu tribes. There was no correlation with the distribution of squamous cell carcinoma of the lower leg, and Kaposi's sarcoma was not seen in an Indian or European during the period under review
Effect of high temperature heat treatments on the quality factor of a large-grain superconducting radio-frequency niobium cavity
Large-grain Nb has become a viable alternative to fine-grain Nb for the
fabrication of superconducting radio-frequency cavities. In this contribution
we report the results from a heat treatment study of a large-grain 1.5 GHz
single-cell cavity made of "medium purity" Nb. The baseline surface preparation
prior to heat treatment consisted of standard buffered chemical polishing. The
heat treatment in the range 800 - 1400 C was done in a newly designed vacuum
induction furnace. Q0 values of the order of 2x1010 at 2.0 K and peak surface
magnetic field (Bp) of 90 mT were achieved reproducibly. A Q0-value of
(5+-1)1010 at 2.0 K and Bp = 90 mT was obtained after heat treatment at 1400 C.
This is the highest value ever reported at this temperature, frequency and
field. Samples heat treated with the cavity at 1400 C were analyzed by
secondary ion mass spectrometry, secondary electron microscopy, energy
dispersive X-ray, point contact tunneling and X-ray diffraction and revealed a
complex surface composition which includes titanium oxide, increased carbon and
nitrogen content but reduced hydrogen concentration compared to a non
heat-treated sample
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